Modern machine tools are operated by numerical controls, which, with the processing of a selected parts program, automatically execute previously defined actions. In this context, to control the drives of the various axes of a machine tool, it is especially significant to correctly determine the position of each drive or of the machine part driven by the drive. Since safety-related functions are often associated with these positions (thus, for instance, tools must not leave a previously defined motion space), it is necessary to reliably design the position detection. In this context, a two-channel position detection or position monitoring is often required for especially critical applications. The signals are processed separately from one another by two separate position transducers, and the resulting positions are compared to one another. In the case of a deviation of the positions, an error in one of the two position-detecting channels can be assumed. A safe state must then be immediately established, for instance, by a forced emergency stop. A two-channel position monitoring of this kind is described, for instance, in European Published Patent Application No. 0 742 505. The outputs of two transducer systems are evaluated in each instance in independent circuits, and the results compared to one another. In many applications, two transducer systems are provided when, for instance, a separate position transducer is used for the speed control and for the positional control of a drive. If, from the side of the control, however, only one position transducer is necessary, then a second position transducer for the two-channel position monitoring signifies additional outlay.
It has previously been proposed, therefore, to operate systems having only one position transducer and, in this context, to design the signal evaluation such that a reliable position monitoring is still possible. German Published Patent Application No. 198 26 875 describes a position transducer DG, the output signals of which are directed to two independent processors CPU1, CPU2. The cabling expenditure this requires (Y—distribution of the signals of the position transducer) is often considerable, however, particularly when (as is customary in machine tools) the drives and the control electronics are situated at a considerable distance from one another.
It is therefore an object of the present invention to provide a device for reliable position monitoring which uses one position transducer and requires little outlay for connecting the position transducer.